The New Class of Dusty DAZ White Dwarfs

Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescopeand the IRAC imager has revealed an infrared excess associated with the whitedwarf WD 2115-560 naturally explained by circumstellar dust. This object is thefourth white dwarf observed to have circumstellar dust. All four are DAZ whitedwarfs, i.e. they have both photospheric Balmer lines and photospheric metallines. We discuss these four objects as a class, which we abbreviate "DAZd", wherethe "d" stands for "dust". Using an optically-thick, geometrically-thin diskmodel analogous to Saturn's rings, we find that the inner disk edges are at>~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This modelnaturally explains the accretion rates and lifetimes of the detected WD disksand the accretion rates inferred from photospheric metal abundances.Comment: 27 pages, 7 figures, ApJ accepte